Fixed if-then-else not activating its propagators.
Merge pull request #104 from JLiangWaterloo/fix-ifthenelse

Fixed if-then-else not activating its propagators.
fix reif

- issue with multiple reifications of same cons
=> only one reification per constraint (if multiple, just reuse the
bool var associated to the cons)
current drawback the opposite cons is generated by default, even for
simple implications (not a big deal)